Ultrafast Electron Dynamics in Polar Liquids and Crystals - Livestream

Electron transfer and charge transport are most elementary processes in liquid and solid condensed matter. Both femtosecond spectroscopy and structure-resolving x-ray methods give insight in electron dynamics at atomic length and time scales. This talk focuses on many-body dynamics of free electrons in water and alcohols, and on soft-mode excitations in polar crystals.

The electric dipole moment of water molecules gives rise to a strong local electric field in the liquid which fluctuates in a time range from tens of femtoseconds to several picoseconds. The fluctuating field induces spontaneous tunneling ionization of water molecules which can be made irreversible by imposing an external directed terahertz (THz) field on the liquid [1]. Time-resolved nonlinear THz spectroscopy [2] allows for mapping charge separation, transport, and localization of the released electron on a few-picosecond time scale. The highly polarizable localized electrons modify the THz dielectric function of water, a manifestation of a highly nonlinear response. The solvated electrons exhibit pronounced polaronic properties, due to many-body Coulomb interactions with a large number of solvent molecules [3].

Soft-mode excitations of polar and/or ionic crystals display a hybrid character with coupled nuclear and electronic motions. Femtosecond x-ray powder diffraction allows for following such correlated dynamics at the atomic level by providing momentary atom positions and charge density distributions [4]. In cubic boron nitride, transverse acoustic two-phonon excitations in the electronic ground state induce a step-like increase of diffracted x-ray intensity, opposite to a Debye-Waller behavior [5]. Transient charge density maps reveal distinctly different length scales of nuclear and electronic displacements and a spatial transfer of valence charge from the interstitial region onto boron and nitrogen atoms. Such findings will be discussed in comparison to other prototypical materials.

Speaker: Thomas Elsaesser, Max-Born-Institute